Sieve-wheel filters with a corresponding back-flushing station belong to the prior art as presented, for example, by DE 103 26 487 [U.S. Pat. No. 7,411,163]. In such sieve-wheel filters, mesh inserts located in the melt conduit are rotated away from the melt conduit via a drive that acts on the sieve wheel and switched with cleaned mesh inserts. The back-flush cleaning station lies as close as possible upstream of the melt conduit in the direction of rotation; during filter operation in the back flush cleaning station, i.e., when the sieve wheel is stationary, the mesh insert located in the back-flushing station is traversed with cleaning agent by a stream generated in the cleaning-agent supply.
The back-flushing station is operated during down times of the filter wheel. Here, a portion of clean melt from the already-cleaned melt flow is fed to the cleaning-agent supply, here a shootback plunger, in the back-flushing station. Upon actuation of the shootback plunger, the melt—which acts as a cleaning agent—is forced through the mesh insert. The mesh insert is traversed by the cleaning agent counter to the usual sieve-flow direction, taking contaminants that have collected on the mesh insert along with it and then guiding them toward an outlet. After cleaning of the mesh insert, it can be rotated a step further and moved to alignment with the melt conduit.
To enable complete cleaning of the mesh insert when the sieve wheel is at a standstill, the size of the back-flush nozzle corresponds at least to the angular dimension of a single rotation step.
In this regard, it is disadvantageous that, given the required size of the back-flush nozzle, large quantities of cleaning agents are required in order to saturate the mesh insert over the required width with the cleaning agents. Particularly at edges of the mesh inserts, insufficient cleaning often occurs because the melt pressure of the back-flushing station is often no longer sufficient there.
DE 39 02 061 [U.S. Pat. No. 5,090,887] discloses a screening device for cleaning plastic melts in which, during rotation of the sieve wheel, cleaning is performed of the mesh inserts located in the back-flushing station. Advanced by the pressure prevailing in the melt conduit, the already cleaned melt flows through the mesh inserts counter to the usual filter-flow direction. A cleaning-agent supply is not proposed. Effective, melt-saving cleaning is not possible.
According to CH 593 786, the already cleaned melt flows constantly through the mesh inserts as a result of the pressure prevailing in the melt conduit. Here, too, no actively functioning cleaning-agent supply is provided. As a result of the continuous cleaning process, an especially large amount of the cleaning agent, i.e. the melt, is wasted.